Fe contents and isotopes in suspended particulate matter of Lancang River in Southwest China.

Iron (Fe) isotope geochemistry in rivers is crucial for comprehending surficial weathering and geochemical cycle mechanisms. Lancang River is an important channel for material transport between the Tibet Plateau and the oceans of Southeast Asia. In this study, Fe contents and Fe isotope (δFe) compositions in the suspended particulate matter (SPM) are investigated to discuss the rock weathering processes in the Lancang River Basin. The δFe values of SPM range from 0.01 ‰ to 0.21 ‰, with an average of 0.12 ‰, close to the average δFe value of continental crust (0.07 ‰). The results indicate that the fractionation of Fe isotopes is limited caused of weathering process in the Lancang River Basin. Due to the interception of dense dams in the middle and lower reaches (1000-2000 m), the dissolved oxygen (DO) values of river water and the Fe contents of SPM remain at a relatively highest level, whereas the δFe values in SPM are more positive. The positive correlation between chemical index of alteration (CIA) values and the Fe contents suggest that Fe in the tributary SPM may represent the weathering degree of their source areas. The increase of DO in the mainstream water may promote the decomposition and dissolution of SPM, thus increasing the contents of Fe in the remaining SPM, and causing slight positive fractionation of Fe in SPM. This study presents a complete analysis of the Fe isotope's potential utility in identifying the source of SPM. In addition, the Fe isotope may represent some alterations encountered by SPM throughout the runoff process.